1. Field of the Invention
This invention relates to a steering shaft for a steering wheel in automotive vehicles. More particularly, this invention is a new steering column and a new method of making the steering column. When an accident involving a motor vehicle occurs, the impact energy is absorbed by plastic deformation of a portion of this collapsible steering shaft.
2. Description of Related Art
As shown in FIG. 11, current collapsible steering shafts with energy absorption mechanisms include generally a corrugated part 10 which varies from larger radius tube 110 to smaller radius tube 190. Yoke 20 and flange 30 play roles as connective parts on both ends 150 of the corrugated tube 10. The corrugated tube's ends 150, and the end portion 250 of the yoke 20 and the flange 30 are joined by welding 50. The corrugated tube 10 is a thin-walled pipe of varying diameter, while the yoke 20 and the flange 30 are made by forging and are of higher rigidity material. As these parts are connected by welding, the strength and rigidity of the joined parts exists discontinuously in the longitudinal direction. This discontinuity causes inefficient absorption of impact energy.
Accordingly, and as a preliminary step in the conception of this invention, forming the corrugated part 10 and the connective parts 20 and 30 from a single material by integral-type forming was considered. As shown in FIG. 10, for example, the tube 100, consisting of the corrugated portion and its two connective ends 200, are formed integrally. Tube 100 is formed continuously by bulge forming and other plastic forming. By using plastic forming, the flow of materials is continuous longitudinally along the part, and the energy absorption performance required of the part is achieved. However, a significant disadvantage of this design is cost. The use of plastic forming of both the corrugated portion and the connective end portions requires adoption of high-grade materials. As a result, the higher cost of materials becomes a serious problem.